Re: Physics 101
Sure! We hop onto the belt, for get that we are on earth, and see that the plane moves forward.
The plane is clearly moving. What's the problem?
cogsncogs said:
Sure! We hop onto the belt, for get that we are on earth, and see that the plane moves forward.
The plane is clearly moving. What's the problem?
Hi Wayne,
Mostly correct. But the mass of the air being expelled out the back of the engine is what creates forward thrust. Jet engines all depend on accelerating a mass of air and gases out the back to function.
Hi eVITAERC,
Why are you taking your point of reference on the belt (Not that it matters)? Using the plane or air or ground as the point off reference is easier on the head. The position of the belt is really not all that important. If you were to hold the plane steady and move the belt, the wheels would turn. Wouldn't they? The holding force on the plane would not be that great and would be equal to the rolling friction of the bearings plus the friction due to tire deformation and whatever losses existed in the belt system.
This is all you have that could possibly resist the thrust of the engines, negating the belt friction (cause that doesn't come from the plane system). There is no way this exceeds the available thrust. Otherwise, aircraft do not leave the earth's surface. Ever.
-Chris
Mostly correct. But the mass of the air being expelled out the back of the engine is what creates forward thrust. Jet engines all depend on accelerating a mass of air and gases out the back to function.
Hi eVITAERC,
Why are you taking your point of reference on the belt (Not that it matters)? Using the plane or air or ground as the point off reference is easier on the head. The position of the belt is really not all that important. If you were to hold the plane steady and move the belt, the wheels would turn. Wouldn't they? The holding force on the plane would not be that great and would be equal to the rolling friction of the bearings plus the friction due to tire deformation and whatever losses existed in the belt system.
This is all you have that could possibly resist the thrust of the engines, negating the belt friction (cause that doesn't come from the plane system). There is no way this exceeds the available thrust. Otherwise, aircraft do not leave the earth's surface. Ever.
-Chris
Hi Chris
Yes! Exactly! The hot expanding gases forced out the back/nozzle of the engine creates an opposing force acting upon the engine which moves it (ideal) in an equal and opposite direction/force of the gasses leaving the rear of the engine. Hopefully I've described the action accurately. It's not the hot gasses pushing against the air the causes the forward momentum of the engine.
😀
Wayne
Edit: A prop plane needs the atmosphere (air) to operate, to push against.
Yes! Exactly! The hot expanding gases forced out the back/nozzle of the engine creates an opposing force acting upon the engine which moves it (ideal) in an equal and opposite direction/force of the gasses leaving the rear of the engine. Hopefully I've described the action accurately. It's not the hot gasses pushing against the air the causes the forward momentum of the engine.
😀
Wayne
Edit: A prop plane needs the atmosphere (air) to operate, to push against.
One more vote for the plane remaining grounded.
Since the conveyer moves at the same speed backwards as the plane moves forwards, there's no net distance travelled - the plane stays still.
Where could any lift generated?
To generate lift, the plane has to move forwards (relative to the ground, not conveyer belt). For it to move forwards, the wheels have to move faster than the conveyer belt - which can not happen given the origainal scenario.
Since the conveyer moves at the same speed backwards as the plane moves forwards, there's no net distance travelled - the plane stays still.
Where could any lift generated?
To generate lift, the plane has to move forwards (relative to the ground, not conveyer belt). For it to move forwards, the wheels have to move faster than the conveyer belt - which can not happen given the origainal scenario.
wintermute said:
Arrgggghhhh 😉 RDfan you have just said what originally made me think the plane couldn't take off, and that line in isolation and your reasoning is I think sound ie what I originally said about it being a relative motion thing..... I think that the problem here is that the postulated scenario is impossible 😉 and it is a purely academic question on relative motion that ignores actual physics
IMO It isn't a question about what would happen in reality it is a question to test whether you read it properly or not 😉
Tony.
My stance on the point of this question is that it is a trick within a trick.
First you go - aha! The plane is gorunded because the conveyer belt is preventing the plane from moving forward
Then you go - oh wait, the plane is operated by a jet, which works on the principle of rocketry. Some momentum is created and therefore must caus the plane to go forward... not to mention conservation of energy.... what the hell? Oh! What happens to the wheel has nothing to do with a JET engine! OMG I am so stupid, of course the plane takes off. I got the trick!
And this is where the original poser of this question comes in and say
Mwahaha got ya! The conveyer belt actually DOES stop the plane form moving relative to air (for all the reasons I mentioned). Then he goes on to analyse the situation with math and whatnot.
Umm at least I hope the original poser got this far...
First you go - aha! The plane is gorunded because the conveyer belt is preventing the plane from moving forward
Then you go - oh wait, the plane is operated by a jet, which works on the principle of rocketry. Some momentum is created and therefore must caus the plane to go forward... not to mention conservation of energy.... what the hell? Oh! What happens to the wheel has nothing to do with a JET engine! OMG I am so stupid, of course the plane takes off. I got the trick!
And this is where the original poser of this question comes in and say
Mwahaha got ya! The conveyer belt actually DOES stop the plane form moving relative to air (for all the reasons I mentioned). Then he goes on to analyse the situation with math and whatnot.
Umm at least I hope the original poser got this far...
RDFan:
Jeez. That's what I thought at first, then I had myself convinced otherwise. Shoulda waited before posting.
Ditto on the Arrrgg from Wintermute.
That ought to teach me to READ the question.
Max
Jeez. That's what I thought at first, then I had myself convinced otherwise. Shoulda waited before posting.
Ditto on the Arrrgg from Wintermute.
That ought to teach me to READ the question.
Max
hmmmmm but!!!!! 😉 the reason I say it is impossible is because in order for the conveyor to start moving the wheels on the plane need to start moving, but for the wheels on the plane to start moving the plane must move forward (edit: relative to the belt, which by the definition of the problem can't happen, as the belt speed always matches the wheel speed)......
so if the plane starts moving the belt is allways playing catchup and is not matching the speed of the wheel at any instant in time... so in reality the plane should be able to accelerate away, and the belt just keeps running faster to match the current speed of the wheels..... but there is always a lag, or the plane couldn't accelerate (edit within the constrains of this fictitious situation) 🙂.....
I'll shut up now 😉
Tony.
so if the plane starts moving the belt is allways playing catchup and is not matching the speed of the wheel at any instant in time... so in reality the plane should be able to accelerate away, and the belt just keeps running faster to match the current speed of the wheels..... but there is always a lag, or the plane couldn't accelerate (edit within the constrains of this fictitious situation) 🙂.....
I'll shut up now 😉
Tony.
...'cause there's ****** all down here on earth.
I fail to see how anyone can still think the plane is grounded. The wheels on a plane rotate for one reason and one alone. They happen to be incontact with ground. Moving the belt just rotates the wheels Once the engines are fired up the belt can do one of three things. It can rotate the wheels forward, backwards or keep the wheels static, but the plane still takes off.
Considering the scenario, I'd wager the belt hardly moves at all.
I fail to see how anyone can still think the plane is grounded. The wheels on a plane rotate for one reason and one alone. They happen to be incontact with ground. Moving the belt just rotates the wheels Once the engines are fired up the belt can do one of three things. It can rotate the wheels forward, backwards or keep the wheels static, but the plane still takes off.
Considering the scenario, I'd wager the belt hardly moves at all.
If you think of the conveyer belt as a restorative force, the belt will move exactly at the same time the wheels move and the plane stays still (relative to the ground) - no catch up is needed.
That holds true if it is the wheels that are providing the torque to move the plane forward, but it isn't the wheels it is the jet, if the jet pushes the plane forward the wheels move as a result, ooops the belt has to catch up 🙂
edit: the wheels can never start turning unless the jet pushes the plane forward, there has to be motion of the jet engine in relation to the surounding air for any movement to start, hence the plane has to move relative to the belt, which by definition can't happen if the belt is allways moving at the same speed as the wheels..... I hated proof by induction at uni, and this is starting to look like a proof by induction 😉
damn I said I was going to shut up now 😉 I can see myself being all red faced at some point in the future 😉
Tony.
Guys, a quick idea that might help clarify things. Assuming the wheelbearing is frictionless, or virtually so, we can replace it with an equivalent...
Mount the rocket/jet engine on a hovercraft, or on a smaller scale, one of those table air hockey pucks, free to move in any direction. Now, does the relative speed of the ground effect the ability to accelerate in any direction? The only constraints now are power v. wind resistance.
Mount the rocket/jet engine on a hovercraft, or on a smaller scale, one of those table air hockey pucks, free to move in any direction. Now, does the relative speed of the ground effect the ability to accelerate in any direction? The only constraints now are power v. wind resistance.
pinkmouse said:
Geez, I could have sworn I posted several times about why the wheel exerts far more force on the plane than friction. At least please try to address those before .
Think of it this way: even when the wheel is free to rotate in a frictionless fashion, is the plane itself allowed to move forward when the wheel isn't allowed to move forward?
eVITAERC said:
But it doesn't. eVITAERC, I suggest you read up on Xeno's Paradox. This is not dissimilar.
Guy's come on..... I'm not actually sure that you are all having a laugh now or if you are being serious?
The ground has absolutely no influence on the planes acceleration unless they are locked from the start.
Whether it’s a jet plane or a prop…..it uses the movement of the air to create forward thrust.
Can I take it we all agree on at least that?
Then all you need to establish is whether or not something will give way before the pane reaches take-off velocity. And the answer is?
As far as I can see it the wheels will be spinning at the same velocity as in normal conditions because the belt only matches the rotational speed….no more no less. Oh and by the way, if someone out there can design a wheel that can rotate to infinite levels…please give me a call…..we can make a fortune!
The ground has absolutely no influence on the planes acceleration unless they are locked from the start.
Whether it’s a jet plane or a prop…..it uses the movement of the air to create forward thrust.
Can I take it we all agree on at least that?
Then all you need to establish is whether or not something will give way before the pane reaches take-off velocity. And the answer is?
As far as I can see it the wheels will be spinning at the same velocity as in normal conditions because the belt only matches the rotational speed….no more no less. Oh and by the way, if someone out there can design a wheel that can rotate to infinite levels…please give me a call…..we can make a fortune!
pinkmouse said:
Are you even remotely serious!? Are you even aware of what Xeno's Paradox is about!? Hello? What does converging infinite series have to do with what force the wheels apply?
Can you at least read up on the things you're talking about? Or at least give a better explaination when you digress into some totally unrelated topic, accuse ignorance, and return to being smug?
Or are we now seeing if the plane will take off at time 1, or at time 2 half of time 1 later, or at time 3 still half of time 2 later, and so on.
Lostcause said:
Okay there must be like a few million posts screaming "THE GROUND HAS NOTHING TO DO WITH THE PLANE AND HOW IT MOVES OMG CAN YOU NOT SEE THAT YOU TARD". We don't need another repeat. What we need is someone to step up and give proof of this from first principles of kinematics.
We can't just assume something that seems reaosnable and become smug about its correctness. This is how people a few thousand years ago reached the conclusion that all the celestial bodies in the heavens revolve aorund the earth, and threatened to burn whoever tried to tell them otherwise for spreading herasay.
once again I (as I'm sure someone said right near the begining of the thread) think the problem is IMO with the question.
here is a thought... is the statement The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation talking about the angular velocity of the wheel and the belt or the linear velocity of the wheel and the belt, ie is it talking about the rotational speed, or speed relative to the surroundings?????
Now if it is talking about angular velocity then for both to be equal I cant see how the plane could be moving forward relative to the suroundings (I'm not trying to say here that the belt prevents the plane from moving forward, its just that I don't beleive it's physically possible for both to be going the exact same speed in opposite directions and have the plane moving forward relative to the surroundings (unless the wheels or belt are slipping).
If however the speed is talking about the speed at which the center of the wheel is moving in a forward direction is being matched by the belt going in the oposite direction, then that is an entirely different situation and the plane can move forward, as the speed of the wheel in the forward direction increases the speed of the belt in the opposite direction matches it, and the angular velocity of the wheel is roughly double what it would be if the plane was running along a standard runway.
Now the first scenario above can't physically happen because the thrust is provided by the jet engine and not by the wheels, so it would be imposible for the jet to be moving forward and the angular velocity of the wheels and belt to be the same.... so either the question refers to the second scenario and the plane will take off, or if it means the first then it is physically impossible given the thrust mechanism and the question is unanswerable if real physiscs is taken into account.
Am I completely insane, or have I got a grasp of the situation 😉
Tony.
here is a thought... is the statement The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation talking about the angular velocity of the wheel and the belt or the linear velocity of the wheel and the belt, ie is it talking about the rotational speed, or speed relative to the surroundings?????
Now if it is talking about angular velocity then for both to be equal I cant see how the plane could be moving forward relative to the suroundings (I'm not trying to say here that the belt prevents the plane from moving forward, its just that I don't beleive it's physically possible for both to be going the exact same speed in opposite directions and have the plane moving forward relative to the surroundings (unless the wheels or belt are slipping).
If however the speed is talking about the speed at which the center of the wheel is moving in a forward direction is being matched by the belt going in the oposite direction, then that is an entirely different situation and the plane can move forward, as the speed of the wheel in the forward direction increases the speed of the belt in the opposite direction matches it, and the angular velocity of the wheel is roughly double what it would be if the plane was running along a standard runway.
Now the first scenario above can't physically happen because the thrust is provided by the jet engine and not by the wheels, so it would be imposible for the jet to be moving forward and the angular velocity of the wheels and belt to be the same.... so either the question refers to the second scenario and the plane will take off, or if it means the first then it is physically impossible given the thrust mechanism and the question is unanswerable if real physiscs is taken into account.
Am I completely insane, or have I got a grasp of the situation 😉
Tony.
eVITAERC
Firstly you have just ignored the point that I and several others made about how your frictionless belt with no mass idea is flawed, and changed your argument to discuss the physics of a rolling wheel.
With the new situation you postulate, then Xeno's paradox is very relevant, as it shows the flaws in simplistic interpretations of how to objects move in time in relation to each other, in this case the wheels and the surface of the moving runway. If you can't see that, then I'm sorry, I'm not sure how else I can explain it within the limitations of this forum. I don't have the time or inclination to dig out my old physics texts and prove it from scratch.
Firstly you have just ignored the point that I and several others made about how your frictionless belt with no mass idea is flawed, and changed your argument to discuss the physics of a rolling wheel.
With the new situation you postulate, then Xeno's paradox is very relevant, as it shows the flaws in simplistic interpretations of how to objects move in time in relation to each other, in this case the wheels and the surface of the moving runway. If you can't see that, then I'm sorry, I'm not sure how else I can explain it within the limitations of this forum. I don't have the time or inclination to dig out my old physics texts and prove it from scratch.
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